The present invention relates generally to a fuel intake system of a multi-cylinder internal combustion engine and, more particularly, to an improvement in distribution of an air-fuel mixture from an idle and low-speed circuit to a plurality of intake passages.
The present invention is applicable to a multi-cylinder internal combustion engine employing a carburetor of a type having at least two intake barrels connected to respective sets of engine cylinders or combustion chambers.
There is known a multi-cylinder internal combustion engine having a fuel intake system including an air-fuel intake passage for each engine cylinder or each group of the engine cylinders. In this type of multi-cylinder internal combustion engine, since the intake passages are independent from each other, there is no possibility that the air-fuel mixture flowing through one intake passage undesirably interferes with the air-fuel mixture flowing through another intake passage and, therefore, the supply of the air-fuel mixture to the engine cylinder or each group of the engine cylinders can advantageously be steadily effected. However, this type of multi-cylinder internal combustion engine requires the employment of a carburetor for each intake passage.
The employment of one carburetor for each intake passage involves a plurality of adjustments to be performed for each carburetor, which are likely to result in variation in performance between these carburetors. In particular, the idle and low-speed fuel circuit necessary to supply an air-fuel mixture during the idling or lowspeed operation of the engine requires a delicate adjustment to provide a proper air-fuel mixing ratio and the failure to achieve this proper mixing ratio in the idle and low-speed fuel circuit is likely to result in fluctuation in engine rotation during the idling, uncomfortable drivability during the low-speed engine operation when the engine is in a vehicle, increased fuel consumption and/or the increased exhaust gas emission.
In order to make it possible to supply air-fuel mixtures of equal mixing ratios to the intake passages, U.S. Pat. No. 3,698,371, patented on Oct. 17, 1972, the invention of which has been assigned to the same assignee of the present invention, discloses the employment of a communication passage extending between the intake passages and having an idle port defined in and positioned intermediately of the communication passage, said idle port having an idle adjustment screw for metering the rate of flow of the air-fuel mixture which is to be supplied into the communication passage from an idle and low-speed fuel circuit. This communication passage has its opposite ends opening into the respective intake passages at a position downstream of the throttle valves in the corresponding intake passages.
The device disclosed in the above mentioned U.S. patent is so designed that, during the idling operation of the internal combustion engine, the air-fuel mixture from the idle and low-speed fuel circuit can be supplied uniformly into the intake passages through the communication passage, thereby substantially eliminating the employment of two idle jet systems, one for each intake passage. This device is advantageous in that no separate and independent adjustment of the idle adjustment screws is required; however, it has been found to have some disadvantages.
In the first place, the manufacture of the device, particularly the machining of the idle port, requires the utmost care and precise adjustments to avoid any possible misalignment of the tapered tip of the idle adjustment screw relative to the idle port which may otherwise result in uneven distribution of the air-fuel mixture into the intake passages. In the second place, in view of the fact that the suction pressure, i.e., the negative pressure, in each of the intake passages varies depending upon the cycle of operation of the piston within the corresponding engine cylinder and the air-fuel mixture itself has a nature tending to cause it to flow from a high pressure region (a low negative pressure region) towards a low pressure region ( a high negative pressure region) due to the effect of a pressure differential like other fluid mediums, the presence of an unbalance in negative pressure between the intake passages which may result from a difference in the opening of the various throttle valves within the respective intake passages results in a larger amount of the air-fuel mixture from the communication passage flowing into one of the intake passages, which has the throttle valve set to a relatively small opening and where a relatively high negative pressure is developed acordingly, than into the other of the intake passages which has the throttle valve set to a relatively large opening and where a relatively low negative pressure is developed accordingly. Therefore, with the device having the construction disclosed in the above mentioned U.S. patent, it is not possible to achieve uniform distribution of the air-fuel mixture into the intake passages through the communication passage.
Although not pertinent to the present invention, Japanese Utility Model Publication No. 52-18342, published on Apr. 25, 1977, the invention of which has been assigned to the same assignee as the present invention, discloses the employment of a by-pass passage extending between the intake passages and having its opposite ends opening towards the respective intake passages at positions downstream of the throttle valves in the intake passages, and a shutter valve installed within one of the intake passages for pivotal movement between closed and opened positions. The device of this Japanese publication is so designed that, only during the deceleration of the internal combustion engine, the shutter valve is brought to the closed position so that substantially the whole amount of the air-fuel mixture flowing in said one of the intake passages can be supplied into the other of the intake passages through the by-pass passage.
The purpose for which the combination of the bypass passage with the shutter valve is employed according to the above mentioned Japanese publication is to substantially eliminate the failure of the air-fuel mixture, supplied into one of the engine cylinders through said other of the intake passages, to ignite, thereby avoiding the possible occurrence of knocking and/or after-burning of exhaust gases.